1. Field of the Invention
This invention relates to an electronic exchange system for a plurality of portable stations such as portable telephone sets and, more particularly to an electronic exchange system capable of suitably controlling a portable station which does not reply to a call request.
2. Description of the Related Art
An electronic exchange system is a product of recent technological development in the field of electronics. It is highly intelligent and capable of executing sophisticated programs under the control of a central processing unit and hence can provide a number of different services other than exchange operations ordinary assigned to it. In recent year, there are available electronic exchange system that can connect not only wired terminals but also portable stations such as radio telephone sets.
In an electronic exchange system capable of exchanging a plurality of portable stations as well as a plurality of wired terminals, cell stations that operate as base stations for the portable stations are provided. The electronic exchange system communicates with a selected portable station via one of the cell stations. The cell stations are typically located at predetermined positions on the ground and provide telephone service for predetermined respective service areas, or calling areas, of the exchange so that any portable station located within that areas can communicate with the electronic exchange system. Consequently, a portable station located within the calling areas can originate a call to or be received a call from a wired terminal connected to the electronic exchange system or a portable station via one of the cell stations to establish a telecommunication channel between them.
Normally, a specific telephone number is assigned to each portable station and the electronic exchange system periodically checks the location of each portable station to find out in which one of the plurality of predetermined service zones, or calling areas, it is currently located. If there is an incoming call to a portable station, the electronic exchange system causes one of the cell stations in which service zone the called portable station is located, thereby calling the portable station. If the portable station responds to the call, the electronic exchange system sets up a telecommunication channel between the calling terminal and the called portable station so that the calling party and the called party can communicate with each other.
In order for the electronic exchange system to detect in which one of the calling areas of the electronic exchange system a portable station is located, the electronic exchange system periodically (or constantly) sends a response request command to each portable station and checks which cell station receives the response of the portable station. In addition, the electronic exchange system can also locate a specific portable station when the portable station sends a connection request by seeing which one of the cell stations receives the connection request. Normally, since a specific telephone number is assigned to each portable station, the portable station to which a response command is sent can be identified if the response from the portable station contains its telephone number. In recent years, however, with a digital system such as a PHS (personal handy phone system), each portable station sends a location registration request each time it moves from one calling area into another so that the electronic exchange system can locate the portable station when it receives the location registration request.
Thus, a conventional electronic exchange system for the cell stations and the portable stations, detects a position of all the portable stations controls data concerning their locations (location data). When a portable station is called, it identifies the calling area where the called portable station is located on the basis of the location data it has and transmits a call signal to the called portable station via the cell station of the identified calling area so that the called portable station is informed of the call.
However, if the called portable station is out of power (turned off) or it is out of the calling areas, that is it has moved out of the entire service area of the exchange system, it cannot respond to the call.
In such a case, the called portable station cannot ringing so that the user of the portable station cannot recognize the incoming call by any means. Consequently, as no telecommunication channel is established between the calling party and the called party, the calling party may eventually give up the call and feel unpleasant since he or she does not use a portable station.
Such a problem is attributable to the fact that the electronic exchange system does not store nor control any data on a call request if the called portable station does not send a response signal in return to the call request or if it cannot communicate with the electronic exchange system. As the electronic exchange system does not store nor control any data on the unsuccessful call request, it cannot send any information on the call request to the called portable station for ever because the exchange system is not adapted to handle such an abortive call if the called portable station does not send back a response signal.
Subscribers owning portable stations may be forced to turn off the terminals for some reason or another. They may forget to turn them on again or the batteries of the power sources of the terminals may run out without being recognized by the owners. More often than not, the subscribers may move out of the entire service area of the exchange system in the course of their daily behaviors. If they end up without recognizing incoming calls made to them, the better part of the benefit of owning such portable stations may be lost.
Thus, if the electronic exchange system calls a portable station and the called portable station does not send back a response signal to the electronic exchange system, the electronic exchange system does not store nor control any data on the unsuccessful call and therefore the subscriber who owns the portable station cannot recognize the call for ever. There may be may occasions where portable stations cannot send back a response signal to the electronic exchange system even if there is a call. So, while a portable station may be beneficial because of its handiness and portability, the high probability of abortive calls can significantly reduce this benefit.
Therefore there is a good reason to expect an electronic exchange system to be so adapted, if there is an unsuccessful call because of an inoperative condition of the called portable station and the terminal restores its operable condition, as to notify the calling terminal of the restored operable condition of the called portable station.
Additionally, conventional exchange systems are not provided with a functional feature of automatically searching and selecting an available channel for a called portable station if a specific channel assigned to it is busy. Since the number of channels of a radio communication is limited, the channel can more often be busy as the number of portable stations allocated to the channel increases so that a call may frequently end up in failure because the channel is busy.
In a private branch exchange system, subscribers of portable stations may normally be seated in the office and not a few wired extension telephone sets may be at hand to make the portable stations useless unless the portable stations and the wired extension telephone sets are made compatible without requiring cumbersome exchange operations. This may provide another reason that reduces the benefit of an electronic exchange system for portable stations. Furthermore, an opposite case in which it is easy to connect calling party with the portable station of the called party in place of the wired extension terminal may be occurred.